As shown in FIG. 1, a typical computer system (10) has, among other components, a microprocessor (12), one or more forms of memory (14), integrated circuits (16) having specific functionalities, and peripheral computer resources (not shown), e.g., monitor, keyboard, software programs, etc. These components communicate with one another via communication paths (19), e.g., wires, buses, etc., to accomplish the various tasks of the computer system (10).
In order to properly accomplish such tasks, the computer system (10) relies on the basis of time to coordinate its various operations. To that end, a crystal oscillator (18) generates a system clock signal (referred to and known in the art as xe2x80x9creference clockxe2x80x9d and shown in FIG. 1 as SYS_CLK) to various parts of the computer system (10). Modern microprocessors and other integrated circuits, however, are typically capable of operating at frequencies significantly higher than the system clock signal, and thus, it becomes important to ensure that operations involving the microprocessor (12) and the other components of the computer system (10) use a proper and accurate reference of time.
One component used within the computer system (10) to ensure a proper reference of time among the system clock signal and a microprocessor clock signal, i.e., xe2x80x9cchip clock signalxe2x80x9d or CHIP_CLK, is a type of clock generator known as a phase locked loop (PLL) (20). The PLL (20) is, an electronic circuit that controls an oscillator such that the oscillator maintains a constant phase relative to the system clock signal. Referring to FIG. 1, the PLL (20) has as its input the system clock signal, which is its reference signal, and outputs a chip clock signal (shown in FIG. 1 as CHIP_CLK) to the microprocessor (12). The system clock signal and chip clock signal have a specific phase and frequency relationship controlled by the PLL (20). This relationship between the phases and frequencies of the system clock signal and chip clock signal ensures that the various components within the microprocessor (12) use a controlled and accounted for reference of time. When this relationship is not maintained by the PLL (20), however, the operations within the computer system (10) become non-deterministic.
FIG. 2 shows a block diagram of a typical PLL (200). The PLL (200) includes a PLL core (250), buffers (212, 214, 216, 218), and a feedback loop signal (221) on a feedback loop path. The buffers (212, 214) increase the drive strength of an output clock signal (215) to supply other circuits of the microprocessor (12 in FIG. 1) with a chip clock signal (217). The buffers (216, 218) buffer the chip clock signal (217) to additional circuits of the microprocessor (12 in FIG. 1). The time delay created by the buffers (212, 214, 216, 218) is accounted for in the feedback signal (221) that is supplied to the PLL core (250).
The PLL core (250) is designed to output the chip clock signal (217), which is a multiple of the system clock signal (201). When the PLL is in xe2x80x9clock,xe2x80x9d the chip clock signal (217) and system clock signal (201) maintain a specific phase relationship. To allow different multiplication ratios, the PLL core (250) may use several xe2x80x9cdivide byxe2x80x9d circuits. A xe2x80x9cdivide byxe2x80x9d circuit reduces the frequency of the input to the xe2x80x9cdivide byxe2x80x9d circuit at its output by a specified factor. For example, the PLL core (250) uses a divide by A circuit (220) with the system clock signal (201), a divide by C circuit (222) with a voltage-controlled oscillator (210) output signal (213), and a divide by B circuit (224) with the feedback loop signal (221).
A phase-frequency detector (202) aligns the transition edge and frequency of a clock A signal (221) and a clock B signal (223). The phase-frequency detector (202) adjusts its output frequency in order to zero any phase and frequency difference between the clock A signal (221) and the clock B signal (223). The phase-frequency detector (202) produces signals that control charge pumps (204, 234). The phase-frequency detector (202), using control signals up (203) and down (205), controls the charge pumps (204, 234) to increase or decrease their output. The charge pump (204) adds or removes charge from a capacitor Cl (206) that changes the voltage potential at the input of a bias-generator (208). The capacitor (206) is connected between a power supply VDD and a control voltage VCTRL (207). The charge pump (234) adds or removes charge from a bias voltage VBP (209) of a bias-generator (208).
The bias-generator (208) produces bias voltages VBP (209) and VBN (211) in response to the control voltage (207). The PLL core (250) may be self-biased by adding the charge pump (234) to the bias-generator (208) bias voltage VBP (209). The addition of a second charge pump (234) allows the removal of a resistor in series with the capacitor (206). A voltage-controlled oscillator (210) produces an output signal (213) that has a frequency related to the bias voltages VBP (209) and VBN (211).
The xe2x80x9cdivide byxe2x80x9d circuits (220, 222, 224) determine the frequency multiplication factor provided by the PLL core (250). The addition of xe2x80x9cdivide byxe2x80x9d circuits (220, 222, 224) enables the PLL core (250) to multiply the system clock signal (201). Multiplying the system clock signal (201) is useful when the chip clock signal (217) must have a higher frequency than the system clock signal (201).
For example, during normal operation, the variables A and C may both be set to one in the divide by A circuit (220) and divide by C circuit (222), respectively. The variable B may be set to 10 in the divide by B circuit (224). The phase-frequency detector (202) aligns the transition edge and frequency of the clock A signal (221) and the clock B signal (223). The phase-frequency detector (202) adjusts PLL core (250) output clock signal (215) frequency in order to zero any phase and frequency difference between the clock A signal (221) and the clock B signal (223). Because the clock B signal (223) has a divide by B circuit (224) that reduces its input frequency by 10, the phase-frequency detector (202) adjusts the voltage-controlled oscillator (210) output signal (213) to a frequency 10 times greater than the clock A signal (221). Accordingly, the chip clock signal (217) is 10 times higher in frequency than the system clock signal (201).
The power consumption of a microprocessor is of concern. Reducing the frequency of the chip clock signal (217) reduces the switching rate of other circuits in the microprocessor (12 in FIG. 1). A low power mode may be entered when there is no activity in the microprocessor for an extended period of time. A slower switching rate typically reduces the power consumption of a microprocessor (12 in FIG. 1).
A change in the frequency of the chip clock signal (217) is accomplished by changing the ratio in the divide by circuits (220, 222, 224). For example, during reduced power operation, the variable A may be set to 16 in the divide by A circuit (220); the variable B may be set to 5 in the divide by B circuit (224); and the variable C may be set to 32 in the divide by C circuit (222). In this example, the frequency of the chip clock signal (217) is 5/16 times the system clock signal (201). Also, the phase-frequency detector (202) updates 16 times less frequently compared to the non-reduced power example above.
Proper operation of the microprocessor (12 shown in FIG. 1) depends on the PLL (200) maintaining a constant phase and frequency relationship between the system clock signal (201) and the chip clock signal (217).
According to one aspect of the present invention, an integrated circuit including a clock path arranged to carry a clock signal; a power supply path arranged to receive power from a power supply; a phase locked loop operatively connected to the power supply path and the clock path, comprising a capacitor, responsive to a phase and frequency difference between the clock signal and an output clock signal, for storing a voltage potential; a leakage current offset circuit operatively connected to the capacitor where the leakage current offset circuit is arranged to adjust the voltage potential; a programmable current source connected to the leakage current offset circuit where the programmable current source comprises a first current source and a first switch arranged to control the leakage current offset circuit; and a combinational logic circuit operatively connected to the programmable current source where the combinational logic circuit is arranged to selectively adjust the programmable current source.
According to one aspect of the present invention, a method for post-fabrication treatment of a phase locked loop including generating an output clock signal; comparing the output clock signal to an input clock signal; generating a current dependent on the comparing; storing a voltage potential on a capacitor dependent on the current; adjusting a leakage current of the capacitor using a leakage current offset circuit responsive to a programmable current source where the programmable current source comprises a first current source and a first switch arranged to control the leakage current offset circuit; and selectively adjusting the programmable current source using a combinational logic circuit operatively connected to the programmable current source.
According to one aspect of the present invention, an integrated circuit including means for generating an output clock signal dependent on an input clock signal; means for storing a voltage potential dependent on a phase and frequency difference between the input clock signal and the output clock signal; means for adjusting a leakage current of the means for storing the voltage potential; and means for selectively adjusting the means for adjusting.